How big? So-o-o-o-o big. Ask the Concord Coalition.

If added to the rates needed for Social Security, the implied payroll tax rate necessary to cover the future costs of both Social Security and Medicare would be 32 percent in 2040 and 44 percent in 2075.

Read the whole thing. Thanks to Bruce Bartlett for the pointer. Bruce seems to be the Washington Post‘s favorite “conservative economist.” For example, see this article.